I. Field of the Invention
This invention relates generally to wireless transmitters. More specifically, the invention relates to power control in a wireless transmitter.
II. Description of the Related Art
The use of wireless communication systems for the transmission of digital data is becoming more and more pervasive. In a wireless system, the most precious resource in terms of cost and availability is typically the wireless link itself. Therefore, one major design goal in designing a communication system comprising a wireless link is to efficiently use the wireless link.
In a system in which multiple units compete for finite system resources, in order for multiple remote units to access common system resources, the wireless link is divided into a series of channels. Channelization can be achieved by one of a variety of well-known techniques such as time division multiple access (TDMA), code division multiple access (CDMA), frequency division multiple access (FDMA) or a combination of these. Each of these channelization techniques, to some extent, limits the frequency bandwidth of the signal transmitted from the remote unit.
In addition, each of these channelization techniques requires the use of power control to some extent to determine the power level at which the remote unit transmits. If the remote unit signal arrives at the hub station at a signal level that is too low, the system performance level may be inadequate to support communication due to excessive errors caused by thermal noise and interference. If the remote unit signal arrives at the hub station at a signal level that is too high, the remote unit generates unnecessary interference to other system users.
FIG. 1 is a schematic diagram illustrating a wireless satellite communication system. A hub station 10 provides digital data transfer capabilities to a plurality of remote units such as a remote unit 14. The hub station sends signals over an uplink forward channel 20 to a satellite 12. The satellite 12 repeats the signal and transmits it over a downlink forward channel 22. The remote unit 14 receives the signal and processes it. The remote unit 14 sends a signal over an uplink reverse channel 24 to the satellite 12. The satellite 12 repeats the signal and forwards it over a downlink reverse channel 26.
A link budget is a design tool used to determine the level at which signals are transmitted offer the system. For example, a link budget is used to determine a nominal level at which the remote unit 14 transmits the reverse link signal over the uplink reverse channel 24 based upon the expected path loss experienced over the uplink reverse channel 24. The satellite 12 may amplify the signal before forwarding it over the downlink reverse channel 26. The link budget estimates the expected path loss of the uplink and downlink reverse channels 24 and 26. In addition, the link budget estimates the expected interference and noise levels introduced by the uplink and downlink reverse channels 24 and 26 as well as noise introduced by the satellite 12 and the hub station 10 such as due to the noise figure of these units. In addition, the link budget estimates the expected variations of these parameters. Using the link budget, a system designer determines a nominal and worst case power level at which the remote unit transmits.
In a communication system which comprises fixed location remote units and which uses a geosynchronous satellite, the path loss of the wireless link channel is fairly consistent overt time. However, weather conditions may vary the path loss to some extent. Depending on the frequency at which the system operates, adverse weather conditions, such as heavy fog, snow, rain or hail, may increase the path loss by several decibels (dB) or more. Therefore, in order to operate efficiently, most wireless systems include a power control loop to control the level at which signals are transmitted and received in the system. For example, the hub station 10 monitors the signal-to-noise ratio of a signal received from the remote unit 14 over the reverse link channels 24 and 26 and notifies the remote unit 14 if the signal-to-noise ratio of the signal falls below a predetermined level. In response, the remote unit 14 increases the power level at which it is transmitting. If the hub station 10 determines that the path loss has decreased, the hub station notifies the remote unit 14 over the forward link channels 20 and 22 and the remote unit 14 decreases the level at which it is transmitting.
In order to reduce the distortion of the reverse link signal, the remote unit 14 is typically designed to comprise a class A power amplifier. Class A power amplifiers provide a high degree of linearity over a substantial range of output power. In order to operate linearly, class A amplifiers require substantially more supply power than the power level of the radio frequency (RF) signals which they produce. A class A amplifier draws the same supply power regardless of the output power which it is producing. Therefore, the size and heat dissipation capabilities of a class A power amplifier may be significant, as well as the cost of their operation. Typically, the size of a class A amplifier doubles for each 3 decibels (dB) extra of power which it is capable of producing. In addition, the cost of the power amplifier increases significantly for each 3 dB extra of power capability. Therefore, it is advantageous to use a link budget to determine the maximum power output level which the remote is required to transmit and to limit the capability of the power amplifier based upon the determination.
FIG. 2 is a graph showing the characteristics of a typical class A amplifier. The horizontal axis represents the RF input power level in units of decibels referred to 1 milliwatt (dBm). The vertical axis represents the RF output power level of the amplifier in the same units. The gain of the amplifier illustrated by curve 32 is approximately 54 dB. For example, when the input drive level is -30 dBm, the output power is 24 dBm. As the input level is increased in 1 dB steps, the output level also increases in 1 dB steps as is characteristic of a linear amplifier. However, at some point, the output power stops tracking the input power on a one-to-one basis. For example, data point 36 on curve 32 represents the point at room temperature at which the gain of the amplifier has decreased by approximately 1 dB to 53 dB of gain. At this point, the input to the amplifier is approximately -19.5 dBm and the output is approximately 33.5 dBm. As the input drive level is increased further beyond the 1 dB compression point, the output of the amplifier does not increase significantly above 34 dBm.
The non-linearities introduced by use of a class A amplifier close to and beyond the 1 dB compression point cause distortion in the modulated signal which causes increased interference levels in adjacent channels for the non-constant envelop signals generated by many modem communication systems. FIG. 3 is a spectrum plot showing the distortion caused when a modulated signal is amplified by a power amplifier which has the characteristics shown by curve 32 of FIG. 2. The horizontal axis represents frequency and the vertical axis represents power level relative to the power level of the modulated signal. The horizontal axis is measured in terms of channels. Channel 1 represents the channel in which the remote unit is operating. Channels 2-8 correspond to other channels in the system. The spectrum plot shown in FIG. 3 is a single side-band plot. However, it may be assumed that the spectrum created is relatively symmetric about the left-most axis as shown in FIG. 3.
Curve 40 represents the spectrum output by the power amplifier when the modulated drive level is -30 dBm. Referring again to FIG. 2, one can see that the power amplifier is quite linear in this region. The modulation bandwidth is limited to less than the bandwidth of the first channel. However, the modulated signal introduces some interference to adjacent channels. For example, in channel 2, the interference level generated is at least 35 dB lower than the intended signal level in channel 1. The interference level continues to drop in channels 3-8.
Curve 42 represents the output of the power amplifier when the input power level is -20 dBm. Referring again to FIG. 2, one can see that operating at -20 dBm is approaching operation at the 1 dB compression point and the amplifier has begun to exhibit increased non-linearity. Due to the non-linearities, the interference level generated by the power amplifier in channel 2 has increased by approximately 10 dB. In addition, the interference level generated in channels 3-8 has also increased. Curves 46, 48, 50, 52 and 54 represent the output of the power amplifier when the input power level is -18, -14, -10, 0 and 10 dBm, respectively. As the input power level increases, the non-linearities continue to increase causing a corresponding increase in the interference level in the adjacent channels. Referring again to FIG. 2, it can be seen that increasing the drive level does not significantly increase the output level once the 1 dB compression point has been reached. For these reasons, it is advantageous to limit the input power level such that the amplifier is operated in the linear region.
The response characteristics of a class A amplifier vary as a function of frequency, temperature and aging. For example, curve 30 of FIG. 2 represents the characteristics of the class A amplifier at a relatively high operating temperature. Notice that the 1 dB compression point has fallen to approximately -22 dBm of input power and 31 dBm of output power. Likewise, curve 34 represents the characteristics of the amplifier significantly below room temperature. The 1 dB compression point has increased slightly with comparison to curve 32. Similar curves could be generated to represent the frequency and aging response of the amplifier.
In addition to the variations of an amplifier over time, frequency and aging, each amplifier produced by a common process exhibits different characteristics due to process variations as well as other factors. FIG. 4 is a graph showing typical variations in operating curves of amplifiers constructed from a common process. Curve 32 represents the average performance at room temperature as also shown in FIG. 2. Curve 60 represents a power amplifier which exhibits the one-sigma low side process variation characteristics. Notice that the 1 dB compression point is approximately -23 dBm and that the maximum output power of the amplifier is approximately 2 dB less than shown in curve 32. Also note that the gain of the amplifier represented by curve 60 is lower than shown in curve 32 even in the linear region. The curve 64 represents a power amplifier which exhibits the one-sigma high side process variation. Note that the 1 dB compression point is approximately -19 dBm and that the gain of the amplifier is higher than the other two curves shown even in the linear region.
In order to design a link budget which is sufficient to operate over process variations, temperature variations, frequency variations and aging, the maximum output power must be limited in view of the worse case 1 dB compression point over all of these factors. Therefore, referring again to curve 60 of FIG. 4, according to the prior art, the maximum output power of the system should be limited to approximately 29 dBm in order to accommodate variations over frequency, temperature and aging of the one sigma low, side power amplifier. Thus, even though the majority of power amplifiers are capable of producing 5 dB more than this maximum at room temperature and some of the amplifiers are capable of producing approximately 7 dB more output power at room temperature, under the prior art, each remote unit's output power is limited according to the worse-case scenario.
Typical prior art remote units use a detector to sense the output power level. When the output power level exceeds the predetermined maximum, the detector alerts the remote unit and the level at which the power amplifier is driven is limited to the current drive level. Because the output of the detector itself can vary according to process gains, frequency, temperature and aging, some margin must also be built into the system to accommodate for these variations. Alternatively, complicated detector calibration mechanisms can be incorporated into factory testing in order to account for some of these variations. In general, however, the maximum output power must be further limited in order to accommodate variations in the detector as well. In conjunction with the limitations imposed by the variations in the power amplifiers, these limitations significantly decrease the efficiency with which the average power amplifier is used during routine operation.
Therefore, there has been a long-felt need in the industry to develop an efficient power control mechanism in a wireless transmitter.